Photovoltaic (PV) modules (also known as solar modules) are used to produce electrical energy from sunlight, offering an environmentally friendly alternative to traditional methods of electricity generation. Such modules are based on a variety of semiconductor cell systems that can absorb light and convert it into electrical energy and are typically categorized into one of two types of modules based on the light absorbing material used, i.e., bulk or wafer-based modules and thin film modules. Generally, individual cells are electrically connected in an array to form a module, and such an array of modules can be connected together in a single installation to provide a desired amount of electricity.
When the light absorbing semiconductor material in each cell, and the electrical components used to transfer the electrical energy produced by the cells are suitably protected from the environment, photovoltaic modules can last 25, 30, and even 40 or more years without significant degradation in performance. In a typical photovoltaic module construction, the solar cell layer is sandwiched between two encapsulant layers, which layers are further sandwiched between frontsheet and backsheet layers, providing weather resistance, UV resistance, moisture barrier properties, low dielectric constant, and high breakdown voltage.
Fluoropolymer films are recognized as useful components in photovoltaic modules due to their excellent strength, weather resistance, UV resistance, moisture barrier properties, low dielectric constant, and high breakdown voltage and can play a role in both wafer-based and thin film modules. For example, fluoropolymer films, such as an ethylene-tetrafluoroethylene copolymer (ETFE) films, may be used as frontsheets in photovoltaic modules instead of the more common glass layer. Challenges associated with using a fluoropolymer film as a frontsheet include providing the desired combination of barrier properties and transparency, as well as providing good adhesion to an adjacent encapsulant layer. For instance, higher transparency will improve solar module efficiency in converting sunlight into electricity, but achieving higher transparency typically requires the use of thinner fluoropolymer films, which reduces strength, weather resistance, UV resistance, and moisture barrier properties. Furthermore, the reduced barrier properties of thinner films can result in more rapid degradation of the encapsulant layer, reducing the overall performance of the module. ETFE films have become the most widely used fluoropolymer materials for manufacture of PV frontsheets due to the excellent adhesion of ETFE to ethylene vinyl acetate copolymers (EVA) which are the most commonly used material for encapsulant layers.
EVA copolymers have been favored encapsulant materials because they are characterized by low melting temperatures, which allows them to readily flow around and seal the solar cell components. However, the low melting temperature properties of EVA copolymers generally necessitate subsequent crosslinking of the polymer so as to impart suitable thermal stability to the resultant photocells. Therefore, alternative materials that exhibit higher thermal dimensional stability without crosslinking have been developed for use in encapsulant layers.
In addition, encapsulant materials have been compounded with silane coupling agents to improve adhesion to fluoropolymer layers. (See U.S. Pat. Nos. 6,963,120 and 6,762,508; U.S. Patent Application Publications 2009/0183773, 2009/0120489, 2009/0255571, 2008/0169023, 2008/0023063, 2008/0023064 and 2007/0267059; U.S. Patent Application No. 61/230,238; European Patent Application 1065731; French Patent 2539419 and Japanese Patent Applications 2000/186114, 2001/144313, 2004/031445, 2004/058583, 2006/032308 and 2006/1690867).
U.S. Pat. No. 6,753,087 discloses a multilayer structure including a fluoropolymer bonded to a substrate. The structure is prepared by treating a surface of the fluoropolymer with a bonding composition that includes an amino-substituted organosilane, contacting a substrate with the treated surface of the fluoropolymer and heating to form a bond. U.S. Patent Application Publications 2008/0023063, 2008/0023064, 2008/0264471 and 2008/0264481 describe solar cells in which one or both surfaces of any of the solar cell laminate layers may be treated with a silane that incorporates an amine function.
U.S. Pat. No. 7,638,186 and European Patent Application Publication EP577985 disclose the use of tetrafluorethylene-hexafluoropropylene copolymers, commonly referred to as FEP, as back sheet layers in photovoltaic modules. PCT Patent Application Publication WO2004/019421 discloses FEP used as a front sheet layer in photovoltaic modules.
Alternatives to ETFE films that exhibit higher transparency and/or better barrier properties would be desirable, particularly for use in flexible solar cell modules where the use of rigid glass is not feasible. Additionally, the alternative materials would desirably have adequate adhesion to encapsulant materials under adverse conditions to enable their use in photovoltaic modules.